As is well understood, increased automotive traffic density, traffic speeds, and use of large, heavy payload automotive vehicles have all aggravated the problem of traffic safety relative to operator performance margins in effecting safe vehicle braking within minimum braking distances and while maintaining vehicle lateral stability. In other words, the operator's actuation and control of the braking system needs to be automatically augmented so as to assure the prevention or reduction of loss of lateral stability, (tendency to jackknife) and skidding (wheel-lock or excessive slipping) and increased stopping distances due to operator skill limitations (resulting in over-control).
As indicated in copending U.S. application Ser. No. 268,070 (now U.S. Pat. No. 3,790,227) owned by Rockwell International Corporation, assignee of the subject invention, there have been a number of prior art systems directed to providing anti-skid brake control by means interposed between the operator control and the brake valves of a fluid pressure operated braking system for variously releasing or otherwise attenuating and re-applying the operator-applied brake pressure.
Such techniques have included various means responsive to combinations of braked wheel acceleration/deceleration and wheel speeds. One approach has compared the slipping wheel velocity with vehicle velocity (non-slipping or synchronous wheel velocity). Such approach may employ a free-wheeling or unbraked wheel to determine such vehicle-velocity or synchronous velocity. Alternatively, zero wheel acceleration under the condition of at least a preselected range of wheel velocity conditions may be employed as indicative of synchronous (non-slip) wheel speed. Still further refinements have employed complex logic with comparisons between the braked wheels at opposite ends of an axle to determine whether a velocity difference exists between such wheels and further, which of such wheels will be employed during such comparison period as a source of braking performance (wheel speed and acceleration) data for system operation.
None of such prior-art approaches have, however, resulted in a system generally useful over a wide range of braking conditions, but instead have demonstrated only limited performance under the restricted range of conditions for which they were adapted, while yet requiring undue complexity. For example, the system disclosed in the above-noted U.S. Pat. No. 3,790,227 monitors the existence of wheel speed differences among a wheel set and determines which wheel performance is to be employed for anti-skid control purposes, thereby adding to the control logic complexity. Also, the system of such copending application controls a restoration of a released brake pressure in response to (after the fact of) a positive wheel acceleration, as to result in a delayed control mode with associated over-control. Such system also employs the determination and storing of a maximum deceleration for control system use, whereby special peak-detecting and memory functions are required, and also employs synchronous wheel speed as a control system criterion. Also, the performance of the above-noted delayed control mode with associated over-control performance is further aggravated by the use of a limited number of preselected discrete values of pressure attenuation (ratios of commanded pressure to available pressure), resulting over-use of pressure dump (zero pressure) and consequent over-control.
In summary, then, the prior art of anti-skid control systems has suffered from over-complexity or limited performance or both.